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Subject: Re: [PATCH v2] fbdev: Detect integer underflow at "struct
 fbcon_ops"->clear_margins.
To:     Daniel Vetter <daniel@ffwll.ch>
References: <adff5d10-fe35-62d4-74c5-182958c5ada7@i-love.sakura.ne.jp>
 <20200715015102.3814-1-penguin-kernel@I-love.SAKURA.ne.jp>
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 <20200716100006.GN3278063@phenom.ffwll.local>
Cc:     Dan Carpenter <dan.carpenter@oracle.com>,
        linux-fbdev@vger.kernel.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        syzbot <syzbot+e5fd3e65515b48c02a30@syzkaller.appspotmail.com>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        George Kennedy <george.kennedy@oracle.com>,
        Jiri Slaby <jslaby@suse.com>,
        Dmitry Vyukov <dvyukov@google.com>
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <b202b74b-6a7b-e2be-2350-72144331303b@i-love.sakura.ne.jp>
Date:   Thu, 16 Jul 2020 20:27:21 +0900
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On 2020/07/16 19:00, Daniel Vetter wrote:
> On Thu, Jul 16, 2020 at 12:29:00AM +0900, Tetsuo Handa wrote:
>> On 2020/07/16 0:12, Dan Carpenter wrote:
>>> I've complained about integer overflows in fbdev for a long time...
>>>
>>> What I'd like to see is something like the following maybe.  I don't
>>> know how to get the vc_data in fbmem.c so it doesn't include your checks
>>> for negative.
>>
>> Yes. Like I said "Thus, I consider that we need more sanity/constraints checks." at
>> https://lore.kernel.org/lkml/b1e7dd6a-fc22-bba8-0abb-d3e779329bce@i-love.sakura.ne.jp/ ,
>> we want basic checks. That's a task for fbdev people who should be familiar with
>> necessary constraints.
> 
> I think the worldwide supply of people who understand fbdev and willing to
> work on it is roughly 0. So if someone wants to fix this mess properly
> (which likely means adding tons of over/underflow checks at entry points,
> since you're never going to catch the driver bugs, there's too many and
> not enough people who care) they need to fix this themselves.

But I think we can enforce reasonable constraint which is much stricter than Dan's basic_checks()
(which used INT_MAX). For example, do we need to accept var->{xres,yres} >= 1048576, for
"32768 rows or cols" * "32 pixels per character" = 1045876 and vc_do_resize() accepts only
rows and cols < 32768 ?

> 
> Just to avoid confusion here.
> 
>> Anyway, my two patches are small and low cost; can we apply these patches regardless
>> of basic checks?
> 
> Which two patches where?

[PATCH v3] vt: Reject zero-sized screen buffer size.
 from https://lkml.kernel.org/r/20200712111013.11881-1-penguin-kernel@I-love.SAKURA.ne.jp

[PATCH v2] fbdev: Detect integer underflow at "struct fbcon_ops"->clear_margins.
 from https://lkml.kernel.org/r/20200715015102.3814-1-penguin-kernel@I-love.SAKURA.ne.jp